CCN Activity, Variability and Influence on Droplet Formation during the HygrA-Cd Campaign in Athens

Aikaterini Bougiatioti,Athanasios Nenes,Alexandros Papayannis,Athina Argyrouli,Stavros Solomos,Stergios Vratolis,Konstantinos Eleftheriadis

doi:10.3390/atmos8060108

Abstract

Measurements of cloud condensation nuclei (CCN) concentrations (cm−3) at five levels of supersaturation between 0.2–1%, together with remote sensing profiling and aerosol size distributions, were performed at an urban background site of Athens during the Hygroscopic Aerosols to Cloud Droplets (HygrA-CD) campaign. The site is affected by local emissions and long-range transport, as portrayed by the aerosol size, hygroscopicity and mixing state. Application of a state-of-the-art droplet parameterization is used to link the observed size distribution measurements, bulk composition, and modeled boundary layer dynamics with potential supersaturation, droplet number, and sensitivity of these parameters for clouds forming above the site. The sensitivity is then used to understand the source of potential droplet number variability. We find that the importance of aerosol particle concentration levels associated with the background increases as vertical velocities increase. The updraft velocity variability was found to contribute 58–90% (68.6% on average) to the variance of the cloud droplet number, followed by the variance in aerosol number (6–32%, average 23.2%). Therefore, although local sources may strongly modulate CCN concentrations, their impact on droplet number is limited by the atmospheric dynamics expressed by the updraft velocity regime.

Highlights

Aerosol indirect effects (AIE) on climate encompass the wide range of interaction of aerosols with clouds, radiation and the hydrological cycle
This study investigates the aerosol-cloud droplet link in a polluted boundary layer, using data collected during the Hygroscopic Aerosols to Cloud Droplets (HygrA-CD) field campaign [18], which took place in Athens, Greece between 15 May and 22 June, 2014
Aerosol and cloud condensation nuclei (CCN) Measurements time series of the observed CCN concentrations are shown in Figure 1, along with the NO2

Summary

Introduction

Aerosol indirect effects (AIE) on climate encompass the wide range of interaction of aerosols with clouds, radiation and the hydrological cycle. The ability of particles to act as cloud condensation nuclei (CCN) and form cloud droplets depends on their size, chemical composition and morphology. These properties, in turn, are affected by the combination of numerous aerosol sources and processing over their atmospheric lifetime. Köhler theory determines the conditions for activating a CCN into droplets through a set of thermodynamic requirements [3]. According to this theory, exposure of a particle to a water vapor supersaturation that exceeds a characteristic (“critical”) level forces the wet CCN to experience unconstrained growth and activate into a cloud droplet. The critical supersaturation, sc , depends on particle size and moles of solute in its aqueous phase

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Results

Conclusion